The present invention relates to a vital microprocessor-based rate decoder for use in onboard mainline railroad and rapid transit ATP (Automatic Train Protection) systems.
An appreciation of the larger system can be gained by reference to co-pending applications Ser. No. 267,070 and 267,218, assigned to the assignee of the present invention, the details of the disclosures of which are incorporated herein by reference. In particular, FIG. 10 of this application depicts the larger system in block form.
The unique rate decoder of the invention incorporates a method for tolerating specific kinds of signal disruption; it further allows the signal disruption to be tolerated to be specified per application.
A fundamental object of the present invention is to design the entire system in such a way that the probability of wrongside failure has a calculable upper bound.
The present invention applies, first of all, the concept of "Tolerance Accumulation", allowing specific types of signal disruption to be tolerated. It further applies the concept of Numerically Integrated Safety Assurance Logic (NISAL) to the design of each of the functions required in the vital rate decoder to allow the calculation of an upper bound to the probability of a wrongside failure.
A vital rate decoder comprises several functions. Each must be designed such that the integrity of the information created or operated on by the function cannot be compromised. This requirement is imposed on the system because of the vital nature of the device.
The functions to be performed are:
a. Interpret the demodulated output of a rate code pick-up coil as a pulse of a particular "rate".
b. Determine a pulse to be valid only if its characteristics, pulse width and duty cycle meet established criteria.
c. Establish a rate as valid only if the immediate history of the received pulses are valid pulses related to that rate and meet established criteria.
d. Output vital data values once each system cycle (Tcyc) which uniquely specify the current established rate.
e. Insure that an established rate remains so only while specified criteria are continually met.